1. Field of the Invention
The present invention relates to a chip type transformer, and more particularly, to a chip type transformer usable as a balun transformer in an impedance converter for converting the impedance of a transmission line of a high frequency circuit having a frequency higher than that of a UHF band, a signal converter for converting a signal of a balanced transmission line and a signal of an unbalanced transmission line into each other, a phase converter, and the like.
2. Description of the Prior Art
The balun transformer is a device to convert the signal of the balanced transmission line and the signal of the unbalanced transmission line into each other. The word "balun" is an abbreviation of "balanced-unbalanced".
As shown in FIG. 11, the balanced transmission line has two signal lines forming a pair, thus transmitting a signal as the potential difference between the two signal lines. Because external noise affects the two signal lines of the balanced transmission line equally, the external noise is offset, and thus it is hard for the external noise to affect the balanced transmission line. Because the circuit of an analog IC is constituted of a differential amplifier, many input-output terminals of the analog IC are of the balanced type, i.e., the input-output terminals input signals to the circuit and output them therefrom as a voltage difference between the two input-output terminals.
As shown in FIG. 12, the unbalanced transmission line transmits a signal as the potential of one transmission line relative to the ground potential (zero potential). As the unbalanced transmission line, a coaxial line and a signal line (micro strip line) provided on a substrate are known.
FIG. 13 is an illustrative view showing an example of a balun transformer which is in the background of the present invention and to which the present invention is applied. A balun transformer 1 has three input/output terminals 2a, 2b, and 2c. In order to convert the signal of the unbalanced transmission line and that of the balanced transmission line into each other, the unbalanced transmission line is connected with the input/output terminal 2a, while two signal lines of the balanced transmission line are connected with the input/output terminals 2b and 2c, respectively. The balun transformer 1 takes out the signal from the unbalanced transmission line, thus supplying the signal to a portion between the two signal lines of the balanced transmission line, or takes out the signal of the portion between the two signal lines of the balanced transmission line, thus supplying the signal to the unbalanced transmission line.
FIG. 14 is a perspective view showing an example of a conventional balun transformer used a core. A balun transformer 1 shown in FIG. 14 includes an 8-shaped core 3 around which a plurality of coils consisting of three coils 4a, 4b, and 4c is wound. The ends of the coils 4a, 4b, and 4c are connected with the input/output terminals 2a, 2b, and 2c, respectively, while the other end of each of the coils 4a, 4b, and 4c is grounded. Thus, the balun transformer 1 shown in FIG. 14 has an equivalent circuit shown in FIG. 15.
However, the balun transformer 1 shown in FIG. 14 has great losses when converting signals in a high frequency band having a higher frequency than that of a UHF band, as shown by the frequency characteristic thereof in FIG. 16. In addition, the balun transformer 1 cannot be miniaturized beyond a certain size.
For these reasons, a balun transformer having a coaxial construction is used in bands having frequencies higher than that of the UHF band.
FIG. 17 is an illustrative view showing an example of a conventional balun transformer having the coaxial construction. A balun transformer 1 shown in FIG. 17 has a center electrode 5. One input-output terminal 2a is connected with one end of the center electrode 5. The other end of the center electrode 5 is opened. Two inner electrodes 6a and 6b are provided around the periphery of the center electrode 5 so as to connect with the center electrode 5 magnetically. The inner end of the inner electrode 6a is connected with the input-output terminal 2b via a drawing line 7a, whereas the inner end of the inner electrode 6b is connected with the input-output terminal 2c via a drawing line 7b. An earth electrode 8 is provided around the periphery of the inner electrodes 6a and 6b so as to sandwich a dielectric material between the earth electrode 8 and the inner electrodes 6a and 6b. One end of the earth electrode 8 is connected with the outer end of the inner electrode 6a, while the other end thereof is connected with the outer end of the inner electrode 6b. Thus, the balun transformer 1 shown in FIG. 17 has an equivalent circuit shown in FIG. 18.
The size accuracy of each component of the balun transformer 1 shown in FIG. 17 affects the characteristic thereof. Thus, it is difficult to apply the balun transformer 1 to a circuit of an apparatus such as a moving radio apparatus required to have a miniaturized construction.